# Propulsion

## Every Action has an Equal and Opposite Reaction?

This is the third of Sir Issac Newton's laws of physics, and one that is very important to space flight. Here's how it works. If you push on anything, it pushes back on you. That's why if you lean against the wall, you don't just fall through it. The wall pushes back on you as hard as you push on it, and you and the wall stay in place. If you throw something, you put more force behind it than just leaning on it, so it pushes back with more force. This is hard to observe, because usually, if you throw something away from you, the friction between you and the floor makes resistance to keep you in place.

But if you take away the friction and try again, you will move away from the thing you threw as much as it moves away from you.

The bigger the push, the bigger the push back. That's why cannons and guns recoil. As the cannon ball flies on one direction, the cannon moves in the opposite direction. If we turn the cannon up on end, it gets a little closer to how a rocket works. The force that pushes the cannon ball down also pushes the cannon up. But since the cannon is bigger than the cannon ball it has more inertia acting to keep it in one place. We would need a larger force to push the cannon a great distance. If we could make a long continuous hot explosion in the cannon, instead of one quick one, we could push the cannon a far distance. The air that is heated would push out the back, pushing the cannon in the opposite direction. This is how jets work as well as how rockets get into space. Remember, because every action has an equal and opposite reaction something will go forward if it is pushing matter behind itself.